1. Field of the Invention
This invention concerns a new method for measuring light transmitted through a semi-opaque object whereby the absorption and scattering events responsible for the attenuation of the traversing light may be identified and quantitated more clearly. This invention has particular application for the early detection of breast cancer, a screening procedure most frequently performed by x-ray means (mammography), light transmission (diaphanography), thermal imaging (thermography), and ultrasound.
2. Description of the Related Art
The non-invasive examination of the interior of materials has been a familiar procedure for many years. This is particularly true for the radiological examination of human tissues using x-ray sources. Many such x-ray techniques have been improved significantly in recent years, both as regards safety and resolution. Safety has been increased by means of greatly improved films, permitting dramatic reductions of radiative doses and the risks associated with exposure to x-rays. Applied to the detection of breast cancer, x-ray examination (mammography) is considered the standard and most successful procedure for detecting early signs of disease, despite a false negative rate often exceeding 20%. False positive results often equal or exceed true positive results.
Another method for detecting breast cancer uses transmission spectroscopy where a bright, quasi-monochromatic light emitter is placed in proximity to the breast surface, and a television recording or digital image is made of tissue illuminated by the light transmitted through it. A similar scan is made at a slightly different wavelength, and the two images are combined by means of an enhancement algorithm. The final image is then examined by a trained interpreter who makes estimates as to probable carcinoma or disease based on various absorption criteria. Such instruments are of the type sold under the tradename "Spectrascan Model 10" manufactured by Spectrascan, Inc. Once again, for this procedure the false negative rate often exceeds 25%, even with a trained interpreter, and false positive rates may be very high.
Mammography, spectral transmission, and ultrasound are often used in conjunction with one another as a means of detecting carcinomas and other lesions by one method when not detected by another. Ultrasound techniques are used primarily to detect nodules or smaller non-palpable masses. It is in the hope of detecting the onset of carcinomas before they are detected by ultrasound that mammography and spectral transmissions have held their greatest promise, yet their most successful applications also have been for the detection of small, non-palpable masses. The combination of methods still produces an unacceptably high level of false negative (and positive) results.
The relatively high levels of false positives referred to above and associated with all of the methods are perhaps even more distressing than the high incidence of false negative results. If all examinations by a certain technique were classified as positive (on the basis of a given screening technique), then the technique would be considered by its proponents as perfect, since all lesions would have been detected. But a huge number of needless biopsies would have been performed, and if every examination required a biopsy, then the screening technique itself would be useless. Girolamo and Gaythorpe, in their recent CRC Critical Review (1984) of Clinical Diaphanography and related measurements (mammography, ultrasound, thermography, etc., present data of many practitioners that show that the number of biopsies performed often exceeds ten times the number of true breast carcinomas found. The diagnostic procedures seem to have some utility, but hardly seem reliable. Indeed, both mammography and diaphanography, currently the most reliable procedures, seem incapable of detecting deep lesions smaller than about 2 mm in diameter.
Since the interpretation of both mammographs and diaphanographs requires trained interpreters, and such training in itself requires a phenomenological correlation between things "seen" and carcinomas discovered by biopsy or other surgical procedure, one should ask the question: Is the information disclosed consistent with the measurement made?
In the case of x-rays (mammography), the uncomfortable and often painful examination procedure requires, for its most useful application, that the examined breast be compressed to make it more uniformly thick to yield "even penetration by the x-rays, less difference in radiographic density of the chest wall area and the nipple, and reduced radiation dose . . . " (Girolamo and Gaythorpe, loc. cit.). Yet, as has already been mentioned, the ability of such measurements to identify early true carcinomas remains very low. Light transmission measurements seem to yield even worse results. Mammograms disclose differences in the absorption of x-rays by various tissue constituents, yet there are many sources for transmission differences that are unrelated to carcinoma of the breast. It is only by training and experience that mammograms may be interpreted properly. But if we seek to detect an abberant cellular morphology of some cells whose extent is often only tens of micrometers, are x-rays whose wavelength are a few nanometers the most suitable radiation source? The mismatch in the wavelength of x-rays with the size of cancerous cells would suggest not. However, the ability of x-rays to penetrate through otherwise opaque material suggests some vague utility for detecting cumulative abberant absorptions of layers upon layers of diseased cells. A nodule of two millimeters diameter detected by x-ray means and confirmed by subsequent biopsy is surely a well-developed carcinoma rather than an early manifestation of cancer. By the time x-ray techniques detect such lesions, the corresponding carcinoma is probably a later manifestation of disease. True, such detection is often earlier than detection by palpation alone and can improve survival statistics, but it surely cannot be called "early detection" which must occur at the cellular level, i.e. at the first occurrence of a cancerous cell. This "early detection" misnomer persists among proponents of mammography.
Light transmission or diaphanographic methods have similar problems. Although light wavelengths in the near infrared provide a better match to the size of mammalian cells, or at least the regions within cells where the cancerous state is confirmed by cytological examination, the tremendous scattering and attenuation of light by tissue makes it difficult, at best, to develop consistent deductions of probable lesions, especially deep in the tissue. Because of the multiple scattering of light within tissue, the detected signals are degraded and accordingly must carry very little information with them about the environment deep within the tissue. The most "spectacular" observations achieved by light transmissions and its variants have been the graphical disclosures of near surface phenomena and features such as veins, implants, cysts, nipple regions, etc.
In reviewing the literature describing x-ray and light measuring techniques for the early detection of breast cancer, I have become aware of a new approach and instrumentation by which light may be used to yield more significant information about the sources of its scattering during its traversal of the breast. Although my discussion has centered primarily upon applications relating to the detection of breast cancer, it will be obvious to those skilled in the art that the method and apparatus may be applied equally well to other tissue, as well as semi-opaque objects not so-related.